A variety of plugs have been designed for sealing hydraulic and other fluid systems, including simple expansion plugs as well as relatively complex plugs. A known expansion sealing plug has a frusto-conical plug body with peripheral U-shaped grooves, an expansion member comprising either an inner wedge pin or a spherical member, and a tapered coaxial bore for receiving the expansion member. The inner wedge pin has a greater average outside diameter and the spherical member has a greater maximum outside diameter than the tapered coaxial bore. The plug body and expansion member are proportioned to be preassembled such that the expansion member is frictionally retained after partial insertion into the plug body. The unexpanded plug is inserted into a counterbore that is reamed or formed with the same taper angle as the plug body and the expansion member is driven axially into the plug such that the plug body is radially expanded to forcibly engage the counterbore.
High system pressures frequently require positive means of retaining the plugs, particularly when the opening to be plugged is large. A large opening results in an increased expelling force exerted on the plug, as a greater surface area is exposed to the system pressure. A commonly used feature of plugs designed to seal large openings under high pressure is to mate male screw threads on the exterior of the plug with female screw threads in the interior of the opening being plugged. The relatively high retention force provided by the mated threads provides the ability to use large plugs that resist expulsion by high system pressures. A difficulty of the use of threaded plugs is that the plugs tend to loosen, allowing the plug to rotate and eventually back out, when the plug and/or the installation site are subjected to vibration.
FIG. 1 shows a prior art sealing plug 10 that uses a threaded cap 12 to secure a plug body 14 in a bore, hole or other opening 16 in a manifold or other part 18. The plug body 14 has a cylindrical shape and carries an O-ring 20 for sealing to the surface of the bore 16. The plug body is inserted into the opening and then secured in the opening by the cap 12 that is externally threaded for engaging internal threads at the outer end of the opening. A lock wire 22 attached to the body 18 is then secured to the cap 12 to prevent the cap from backing out of the hole, as might otherwise occur as a result of vibration.